The present invention relates to phosphor compositions and methods for making them which are useful for detection of high energy radiation in x-ray computer tomography systems and methods. More particularly, it relates to a new and improved rare earth oxisulfide-based phosphor composition exhibiting excellent luminous intensity and markedly reduced afterglow.
Detectors for the detection of high-energy radiation can be constructed of a phosphor and a photodiode or, respectively, a photomultiplier. Such detectors are widely employed in nuclear medicine and in X-ray diagnostics. The phosphor thereby has the job of absorbing the high-energy radiation and of emitting visible light as a consequence of this absorption. This visible light can be detected by a photosensitive element, for example a photodiode, a photomultiplier or a light-sensitive film.
In modern radiation detectors as employed, for example, in X-ray computer tomography, phosphors having extremely low afterglow or persistence are required in order to achieve an adequately high pulse frequency for the X-ray pulses. A widespread phosphor is thallium-doped cesium iodide CsI:T1, which, for example, still exhibits an afterglow intensity of approximately 10.sup.-2 through 10.sup.-3 of the initial light intensity 20 msec after the high-energy radiation has been shut off. However, phosphors whose afterglow decays significantly faster are required for newer, next-generation radiation detectors.
The oxisulfides of the rare earth elements are promising phosphors for use in modern radiation detectors. DE 36 29 180 C2 discloses a method for the manufacture of a phosphor ceramic having the general composition (Ln.sub.1-x-y M.sub.x Ce.sub.y).sub.2 O.sub.2 S:X with Ln=Gd, La or Y; M=Eu, Pr or Tb and X=F or Cl with 0&lt;x, y&lt;1. The pigment powder employed as starting material is placed in a vacuum-tight metal container and compressed into a ceramic by isostatic hot-pressing. The phosphors obtained in this way, however, exhibit an undesirably pronounced afterglow.
J. Electrochem. Soc., Vol. 136, No. 9, September 1989, pages 2713 and following, proposes a phosphor ceramic of a rare earth oxisulfide doped with cerium to reduce the afterglow. However, a colored phosphor ceramic whose light yield is reduced is obtained due to the cerium additive. The phosphor is thus degraded in terms of another important characteristic.